[Bldg-sim] PTHP Baseline outperforming VAV

Paul Riemer Paul.Riemer at dunhameng.com
Tue May 24 08:40:06 PDT 2011


Marcus,
Compared to packaged DX equipment, an air cooled chiller has two fundamental efficiency disadvantages: extra heat transfer approach and the pumping energy of the water.

Load reductions always help, but to hone in a comparison of the efficiencies of these mechanical systems, I also suggest you look closely at:

-          the COP values and their corresponding rated ambient temperatures in the referenced standards and on your equipment literature

-          compressor types, sizing, and curves

-          maybe, thermal storage

Good luck,

Paul Riemer, PE, LEED AP
Associate / Mechanical
DUNHAM

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Bishop, Bill
Sent: Tuesday, May 24, 2011 9:57 AM
To: Marcus
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] PTHP Baseline outperforming VAV

Hi Marcus,

I agree with your selection of PTHP for the baseline, since your project is a hotel, which is specifically listed in the notes to Table G3.1.1A as being a residential building type.
I am not surprised that you are showing an energy penalty versus an ASHRAE 90.1 PTHP - the cooling and fan energy is relatively low for those units.

As James said, make sure you are accurately modeling your fan and pumping energy in the proposed system. Unfortunately, there are no pressure drop adjustment allowances for Systems 1 & 2.
The maximum allowable SHGC (0.25) is already pretty low for Abu Dhabi, so your "better windows" might not have a lower SHGC than the value required for the baseline, and you will not get a solar load reduction in the proposed model.

Since your baseline cooling efficiency is dependent on the system capacities, make sure you are not using too large of a baseline cooling COP. (It should actually be calculated separately for each baseline system.) Using the efficiency equation from Table 6.8.1D, assuming 400 cfm/ton air flow rate and removing the fan power energy from the efficiency, I calculate COP = 2.94 for a 15,000 Btu/hr unit (for example). There is a separate heating COP but your heating load is negligible per your report below.

Regards,
Bill

[cid:image001.jpg at 01CC19FC.DBFE4970]

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Cheney
Sent: Monday, May 23, 2011 9:55 PM
To: James V Dirkes II, PE
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] PTHP Baseline outperforming VAV

Hi Marcus,

Also pay attention to better glass in the proposed design. In a cooling-dominated building, better window means the heat can not easily escape from the building. You will not easily get credit from improving your window here.

Regards,

Cheney

LinkedIN @ http://ca.linkedin.com/pub/yu-cheney-chen/27/637/72b

On Mon, May 23, 2011 at 3:18 PM, James V Dirkes II, PE <jim at buildingperformanceteam.com<mailto:jim at buildingperformanceteam.com>> wrote:
Dear Marcus,

*         It seems that your Baseline system, at >5 floors, should be VAV with a chiller instead of a heat pump (using LEED 2009).

*         As modeled, your baseline DOES have a notably higher COP, so it may well use less energy.  Part load performance will have a significant effect, so you may want to check and compare  those curves.

*         Also check pump  delta T and total pressure drop.  If you continue with a non-chiller system, pump energy is  a significant factor.  If your Baseline becomes a chiller system, pump energy will probably become similar.

*         Check fan total pressure drop for each system since your fan power is significantly larger in the Proposed system.

*         In the US, it is common to apply occupancy sensors on a LEED project and ASHRAE 90.1 allows a 10% power reduction when these are used.  You are not taking advantage of that and may want to do so.

The Building Performance Team
James V. Dirkes II, P.E., BEMP , LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653<tel:616%20450%208653>

From: bldg-sim-bounces at lists.onebuilding.org<mailto:bldg-sim-bounces at lists.onebuilding.org> [mailto:bldg-sim-bounces at lists.onebuilding.org<mailto:bldg-sim-bounces at lists.onebuilding.org>] On Behalf Of Marcus
Sent: Monday, May 23, 2011 4:24 PM
To: bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Subject: [Bldg-sim] PTHP Baseline outperforming VAV

Hi all,

Having some difficulty in an App. G model for a 6-floor 14,000 m2 GFA Hotel in zone 1B (Abu Dhabi). Assuming standard design for each, should a proposed VAV system out-perform window AC units (the baseline)?

Proposed:
VAV with 2 Air-cooled centrifugal chillers, chiller COP = 3.1 (2.8 if condenser fan energy included)
Envelope - Better windows decrease heating demand by 8% rest of envelope is minimum required for compliance

Baseline:
System Packaged terminal heat pump, COP = 3.52

Using Energyplus v6.0 the results are;

Site Energy in End Uses                  BL   Proposed   COP5
                                         MWh     MWh     MWh
                       Space Cooling    747.5  1052.0   715.6
                      Heat Rejection      0.0     0.0     0.0
                       Space Heating      1.3     0.1     0.1
                               Pumps      0.0    60.0    60.0
                     Fans - Interior     42.2   137.7   137.7
                     Fans - Car park      0.0     0.0     0.0
                   Interior Lighting    530.5   530.5   530.5
                   Exterior Lighting      0.0     0.0     0.0
               Service Water Heating      0.0     0.0     0.0
        Receptacle/Process Equipment    146.3   146.3   146.3
               Data Centre Equipment      0.0     0.0     0.0
            Elevators and Escalators      0.0     0.0     0.0
                   Total Site Energy   1467.9  1926.6  1590.2

I have the energy for the baseline, the proposed with COP = 3.1, and then trying COP = 5. It seems that even if we drastically increase the COP of the chiller plant in the proposed VAV system, we still can't get any energy savings. Is this realistic, an artifact of the simulation, or just an error on my part?

Any guidance would be appreciated,

Marcus
--
Marcus Jones, LEED AP, M.Sc.
Freelance energy consultant
Vienna, Austria

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